The Collaborative International Dictionary of English v.0.48:

Strengthless \Strength"less\, a.
   Destitute of strength. --Boyle.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "strengthless":
   anemic, asthenic, bloodless, chicken, cowardly, debilitated,
   drooping, droopy, dull, effete, etiolated, faint, faintish, feeble,
   flabby, flaccid, floppy, gone, gutless, imbecile, impotent,
   languid, languorous, limber, limp, listless, lustless, marrowless,
   nerveless, pithless, pooped, powerless, rubbery, sapless,
   sinewless, slack, soft, spineless, unhardened, unnerved, unstrung,
   weak, weakly